Why is java.lang.Long not persistable?

I am trying to query for a list of ids of type Long in GAE/JDO. And I'm getting the following exception when I call detachCopyAll() on the result set.

org.datanucleus.jdo.exceptions.ClassNotPersistenceCapableException: The class "The class "java.lang.Long" is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data/annotations for the class are not found." is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data for the class is not found.
at org.datanucleus.jdo.NucleusJDOHelper.getJDOExceptionForNucleusException(NucleusJDOHelper.java:241)
at org.datanucleus.jdo.JDOPersistenceManager.jdoDetachCopy(JDOPersistenceManager.java:1110)
at org.datanucleus.jdo.JDOPersistenceManager.detachCopyAll(JDOPersistenceManager.java:1183)
...

I can query for a list of User objects and detach them just fine. I expected all primitive wrapper classes like Long to be persistable. What am I doing wrong? Below is the code I'm working with.

@PersistenceCapable(identityType=IdentityType.APPLICATION, detachable="true")
public class User
{
    @PrimaryKey
    @Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY) 
    private Long id;

    private String email;
}

@SuppressWarnings("unchecked")
public static List<Long> getUserKeys(String email)
{
    assert email != null;
    List<Long> keyList = null;
    PersistenceManager pm = null;
    Query query = null;
    try {
        pm = PMF.get().getPersistenceManager();    
        query = pm.newQuery("select id from " + User.class.getName());
        query.declareParameters("String emailParam");
        query.setFilter("email == emailParam");
        List<Long> resultList = (List<Long>) query.execute(email);          

        // next line causes the ClassNotPersistenceCapableException
        keyList = (List<Long>) pm.detachCopyAll(resultList);
    }
    finally {
        if (query != null) query.closeAll();
        if (pm != null) pm.close();
    }

    return keyList;
}

    List<Long> resultList = (List<Long>) query.execute(email);          

    // next line causes the ClassNotPersistenceCapableException
    keyList = (List<Long>) pm.detachCopyAll(resultList);

I don't understand what you are doing here. A List<Long> does not have to be detached. You'd want to detach instances of your User entity class, but a Long is a Long, and you can just do whatever you need to do with the resultList.

The error message is confusing, but just caused by Long not being an entity class.